1. Field of the Invention
The present invention relates generally to solid state image sensors and more particularly is directed to an improvement of a solid state image sensor previously proposed by the same assignee by which blooming suppressing effect can be increased and the dynamic range can be raised.
2. Description of the Prior Art
The outline of the previously proposed solid state image sensor will be described with reference to FIG. 1. As shown in FIG. 1, this solid state image sensor is formed of a photosensitive region 3 having a group of plural vertical shift registers 1 each formed of a charge transfer device, for example, a CCD (charge coupled device), a plurality of light receiving areas 2, each of which is located between adjacent vertical shift registers 1 and correspond to each picture element and is capable of accumulating a charge. Read-out gate sections 7 are provided, each of which transfers a signal charge of each light receiving area 2 to one side thereof, namely, the corresponding vertical shift register 1. Also provided are a memory or storage section 5 having a group of vertical shift registers 4 similarly formed of CCDs, each of which is electrically connected to one end of each vertical shift register 1 within the photosensitive region 3 and a horizontal shift register 6 of the same CCD, which is coupled to the storage section 5. In the vertical shift register 1, the transfer electrodes of the transfer section corresponding to each light receiving area 2 are connected together at every other or every field (odd fields or even fields) to which two-phase clock voltages V.sub..phi.A and V.sub..phi.B are applied. The electrode of each read-out gate section 7 is formed the same as that of, for example, the transfer electrode of the corresponding vertical shift register 1 receives the same potential as that applied to the transfer electrode of the vertical shift register 1. Reference numeral 8 designates a channel stop region which is formed to separate each vertical line and each light receiving area 2. Portions other than the light receiving area 2 are all masked to shield them from light.
With such a solid image sensor, the signal charge accumulated in the light receiving area 2 during the light receiving and accumulating period is transferred from the light receiving area 2 to the vertical shift register 1 via the read-out gate section 7 during every field during the vertical blanking period, then are transferred therefrom to the storage section 5 at higher speed and then are stored therein. Thereafter, the signal charge of one horizontal line each is transferred from the storage section 5 to the horizontal shift register 6 during each horizontal scanning period and then the signal charge of one picture element is read out from the output terminal sequentially. In this solid state image sensor, the signal charge received by the light receiving area 2 is read out to the light-shielded vertical shift register 1, then is transferred from the vertical shift register 1 to the storage section 5 at high speed and then the signal charge stored in the storage section 5 is supplied through the horizontal shift register 6 during each horizontal line. Therefore, there is an advantage in that the picture quality can be prevented from being deteriorated by so-called smear.
The ordinary drive method of this solid state image sensor will be described with reference to a timing chart of FIG. 2. In the figure, reference letter T.sub.0 designates the vertical blanking period and V.sub..phi.A and V.sub..phi.B two-phase clock voltages applied to the transfer electrodes of the vertical shift register 1.
As shown in FIG. 2, in the field to be read out, the vertical shift register 1 is driven in response to transfer clock signals .phi..sub.A1 and .phi..sub.B1 during the first period of the corresponding vertical blanking period T.sub.0 to thereby transfer or discharge undesired charge remaining in the vertical shift register 1 therefrom (this transfer of signal charge is called the discharge-transfer). During the next period after the discharge-transfer of the undesired charge, a read pulse P.sub.1 is applied to one of the transfer electrodes of the vertical shift register 1 to open the read-out gate section 7 corresponding to the light receiving area 2 of the field to be read whereby to transfer the signal charge from the light receiving area 2 to the vertical shift register 1 (this transfer of signal charge is called a read-out transfer). During the subsequent period, the transfer clock signals .phi..sub.A2 and .phi..sub.B2 are applied to the vertical shift register 1 to thereby transfer the signal charge from the vertical shift register 1 to the storage section 5 (this transfer is called a frame-shift transfer). Thereafter, the solid state image sensor operates in the light receiving and accumulating period. Meanwhile, during the light receiving and accumulating period (the period other than the vertical blanking period during which the signal of the opposite field is read out), the vertical shift register 1 in the photosensitive region 3 is used as an overflow drain region. In this case, during, for example, such period, a constant voltage is applied to the vertical shift register 1 to extract the charges which overflow from the light receiving area 2 to the vertical shift register 1 over the read-out gate section 7 during the light receiving and accumulating period. Here, the opposite field, if it is a one frame light receiving and accumulating period of the odd field (or even field), represents the opposite even field (or odd field). When the vertical shift register 1 is used as the overflow drain region, in addition to supplying the constant voltage to the vertical shift register 1, it is also possible that the vertical shift register 1, for example, is driven by the clock signal to transfer and extract the overflow charge.
Thus, it can be assumed that in the solid state image sensor of such system the vertical shift register 1 can process sufficient charges. Then, during the vertical blanking period of the opposite field (the duration of time required for the so-called discharge transfer, the read-out transfer and the frame-shift transfer) during the one frame light receiving and accumulating period, the vertical shift register 1 is not utilized as the overflow drain region so that limiting of the blooming control is determined by the amount of light received within the vertical blanking period of the opposite field and which overflows from the light receiving area 2. The prior art for limiting the conventional blooming control will be described with reference to broken lines 11 and 12 in FIGS. 3A and 3B. FIG. 3A is a timing chart showing a clock voltage V.sub..phi. (V.sub..phi.A and V.sub..phi.B) applied to the vertical shift register 1 (the same clock voltage is applied to the read-out gate section 7). FIG. 3B shows the potential in the light receiving area 2 which charges depend on the amount of light signal, namely, the amount of signal charge wherein reference letters V.sub.R0, V.sub.S and V.sub.M respectively represent the potential during the read-out operation of the read-out gate section 7, upon receiving the light and during the vertical blanking period through which the signal charge of the opposite field is read out. In the prior art, since the voltage V.sub.100 applied to the read-out gate section 7 during the one field light receiving and accumulating period is constant as shown by the broken line 11 in FIG. 3A during the periods other than during the vertical blanking period of the opposite field, the potential of the read-out gate section 7 becomes the constant potential V.sub.S during the periods before and after the vertical blanking period of the opposite field. Accordingly, when a light of high intensity is received by the light receiving area 2, the potential of the light receiving area 2 becomes the potential V.sub.S immediately before the vertical blanking period of the opposite field. Therefore, if Q.sub.M is taken as the amount of that can be accumulated during the vertical blanking period of the opposite field (duration of time required for the discharge transfer, the read-out transfer and the frame-shift transfer), namely, a so-called tolerance limit amount of charge, an amount of light 1, is the limit of the blooming control as shown by the broken line 12 in FIG. 3B.
Thus, if the amount or charge to be processed is taken as Q.sub.S and the tolerance limit amount of charge is taken as Q.sub.M, under the condition that the potential of the read-out gate section 7 during the one frame light receiving and accumulating period is constant, the blooming suppressing effect K of the previously proposed solid state image sensor is determined by the following expression. ##EQU1##